The new wave of violence in western Iraq has created a "toxic brew" of hardship that has ballooned the country's crippling refugee crisis by more than 50 percent in just a few days, international aid groups said Thursday.

Just this week, an estimated half-million people have fled the city of Mosul, which was overrun by suspected militants of the Islamic State in Iraq and the Levant, also called ISIL or ISIS, international aid workers told NBC News. Most, about 300,000, are pitching up in Erbil province in the north, according to the office of the U.N. High Commissioner for Refugees, with the rest flowing elsewhere inside Mosul's Nineveh province.

That's in addition to almost 1 million refugees whom the Iraqi government and international aid organizations have already identified, many of them from neighboring Anbar province, where ISIS and other militant groups have fiercely battled the Iraqi army since January.

Further complicating matters: Several hundred private international contractors in the country to help rebuild infrastructure and governmental programs were being evacuated as ISIS advanced, U.S. officials told NBC News.

The State Department called the development a "critical problem" and said it was providing $12.8 million to international groups to help pay for food, shelter and medicine.

"We urge other donors to help meet the critical needs" of displaced Iraqis, spokeswoman Jen Psaki said Thursday.

On Thursday, NBC News saw one car packed with 13 people trying to leave Mosul. Its occupants said they heard gunshots near their home and had simply had enough.